Atrial fibrillation is a condition that affects roughly 2 million people in the US each year. The disease manifests itself through disorganized firing of electrical signals in the atria of the heart and thereby altering the ability of the heart to contract in an efficient manner. The exact etiology of the disease is manifold, but the treatment is usually through intervention to alter the conductive pathways of the heart.
One approach that is in common use today is ablation of the conductive pathways around the pulmonary vein to prevent the disorganized firings. Several approaches have been used for these treatments and include the use of radio-frequency ablation, cryogenic ablation, laser ablation, microwave ablation and high intensity focused ultrasound ablation. In the case of radio-frequency ablation, a catheter is typically inserted into the femoral vein and guided under X-ray fluoroscopy across the septal wall towards the vicinity of the pulmonary veins. The radio-frequency tip of the catheter is then placed against the wall of the heart and activated. The resulting heat is absorbed by the tissue and results in tissue coagulation and scarring thereby reducing the conductive efficiency of the tissue. This is performed at many spatial locations until the necessary tissue is destroyed. A procedure can take up to 8 hours to perform. A similar procedure is currently in use for cryogenic ablation.
The use of ultrasound as an alternative ablation energy source is advantageous for several reasons. The first is that the heating spot is away from the tip of the catheter—this allows for optimizing damage to the correct conductive cells which are not on the inside of the vein but rather on the epicardial surface. By applying heat to the tissue not in contact with the blood pool, one also minimizes the possible complications due to blood clot formation associated with the heating source. Another advantage with incorporating specially designed ultrasonic sources is that multiple locations can be treated at the same time. This dramatically reduces the treatment time from 6-8 hours to 1 to 2 hours, thereby minimizing the amount of time the patient has to be under anesthesia and increasing workflow efficiency. Furthermore, the opening of a hole in the septal wall is no longer required in all cases, as the ultrasonic transducer can have a focus of several centimeters and the absorption of ultrasound by blood is very small.
US 2006/0058711 A1 by ProRhythm, Inc. has proposed a specific solution for a high intensity focused ultrasound device that can be mounted on the end of a catheter. Their device allows the focusing of sound into an annular ring that coincides with the ostium of the pulmonary vein. Through this mechanism, it is possible to simultaneously heat and destroy tissue circumferentially surrounding the vein. The benefit is realized through reduced procedure time. The ProRhythm design is based on a reflective inflatable balloon that results in ultrasound signals reflected in such a way to focus at a specific location in the acoustic field. The drawback to this design is that a separate catheter with a fixed inflatable balloon has to be used depending on the size of the pulmonary vein. This complicates therapy planning, results in variable outcomes, and also introduces both manufacturing and inventory complications.
Another example of an ultrasonic hyperthermia device can be found in the paper “Ultrasonic Phased Arrays with Variable Geometric Focusing for Hyperthermia Applications” by Y. J. Yoon and P. J. Benkeser, IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, Vol. 39, No. 2, March 1992, pp. 273-278. The device described in this paper obtains 3D adjustable ultrasonic focusing by a combination of controlling a volume of liquid (e.g. silicon oil) in a spherical liquid lens, and electronic focusing by controlling electric phases of a linear phased array. In the liquid lens ultrasonic beams are transmitted through the lens boundary and focused in the transmission through the lens boundary. However, the combined use of a phased array and an adjustable liquid lens to obtain 3D focus control makes this ultrasonic device rather complicated.